Evidently a metaphor of this kind is quite different in origin from such a phrase as 'smouldering' discontent; the former we may call, for want of a better word, 'natural' metaphor, as opposed to the latter, which is artificial.

It is a hardy Rose also, in color so darkly red as to be almost black, -- a warm red, less crimson than scarlet, glowing with a kind of smouldering splendor, with only two rows of petals round a centre of richest gold.

As in the case of oxidation by palladium in air, the hydrocarbon appeai*s to undergo a kind of "smouldering" which changes rather suddenly as the temperature rises to a condition of much more intense oxidation.